An Order-in-Council is a type of legislation in the United Kingdom and certain Commonwealth countries which is formally made in the name of the Queen (or the Governor-General acting on her behalf) by the Cabinet, which is a committee of the Privy Council (the Queen-in-Council or the Governor-in-Council). However, two different types of Order-in-Council exist, in the United Kingdom at least. The first type is an exercise of the Royal Prerogative: as such it is primary legislation and does not depend on any statute for its authority, although it may be overridden by an Act of Parliament (Council of Civil Service Unions v. Minister for the Civil Service [1985] 374 at 399, per Lord Fraser of Tullybelton ). This type has become less common with the passage of time, as statutes encroach on areas which used to form part of the Royal Prerogative.

However, a distinction must be made between Orders-in-Council whereby the Queen-in-Council exercises the Royal Prerogative, and Orders-in-Council made in accordance with an Act of Parliament. In this second case, an Order-in-Council is merely another form of statutory instrument regulated by the Statutory Instruments Act 1946 (in the U.K.), albeit subject to more formalities than a simple statutory instrument. This kind of Order-in-Council tends to be reserved for the most important pieces of subordinate legislation, and its use is less common than before. Like all statutory instruments, they may either be annulled in pursuance of a resolution of either the House of Commons or the House of Lords ('negative resolution procedure') or require to be approved by a resolution of either House, or, exceptionally, both ('affirmative resolution procedure'). That said, the use of Orders-in-Council has been extended recently, as the Scotland Act 1998 provides that draft Orders-in-Council may be laid before the Scottish Parliament in certain circumstances in the same way as they would have been laid before the Westminster Parliament.

Matters which still fall within the Royal Prerogative, and hence are regulated by (Prerogative) Orders-in-Council include dealing with servants of the Crown, such as the standing orders for civil servants, making appointments in the Church of England and dealing with international relations.

Whilst the Northern Ireland Assembly remains suspended, much Northern Ireland legislation is made by Order-in-Council. This is done under the various Northern Ireland Acts 1974 to 2000, and not in virtue of the Royal Prerogative.

In the rest of the Commonwealth they are used to carry out any decisions made by the Cabinet and the executive that would not need to be approved by Parliament.

Although the Orders are nominally made by the Queen or her representative, her assent is now purely formality. What actually happens is that the Lord President of the Council (a cabinet minister) reads out batches of Orders-in-Council, which will have been written by the government, in front of the monarch or representative, who, after every couple of orders, says 'Agreed'. They then pass into law, where they are fully effective.

Traditionally, Orders-in-Council are used as a way for the Prime Minister to make political appointments, but they can also be used to issue simple laws as a sort of decree. Often in times of emergency a government may issue legislation directly through Orders-in-Council, forgoing the usual parliamentary procedure in accordance with the Defence of the Realm Act (now repealed) and assorted other emergency powers legislation. However, most Orders of this sort are usually eventually formalized according to the traditional lawmaking process, if they are not revoked at the end of the emergency.

Occasionally Orders are used to effectively reverse court decisions without involving parliament, for example to prevent the Ilois islanders returning to Diego Garcia from which they had been unlawfully exiled.